"What's alarming about that is that we do have people reporting to the authorities, but then they are not being properly investigated.

"The real concern that I have is that in 2015 we had 986 cases involving minors, yet the official figures show that there's only 928 actual crime recorded incidents. So that means ... potentially the cases involving minors are not being investigated properly.

"The fact that the number of crimes recorded hasn't even reached the number of minors actually tells me that all these cases ... they are not being investigated properly because until they are recorded properly, and there are the resources put into it to see what the circumstances are behind that, we can't be satisfied that we can say it is, or it isn't, a crime of modern slavery," the commissioner told BBC Radio 4's Today programme.

The number of potential slavery victims being brought to the UK has risen from below 1,000 in 2011 to over 3,200 last year, but Mr Hyland said Government estimates suggest the real figure could be 13,000.

But the commissioner insisted progress is being made in achieving convictions.

"There is some good news, last year there were far more convictions, and this year we have already passed the amount of convictions for the whole of last year. So, last year there was 130 convictions, and we have already gone past that number in the first six or seven months of this year, so it is going in the right direction," he said.